Air knives are known for directing elongated air curtains for various purposes such as drying, cooling, or cleaning objects placed in stationary opposition to, or conveyed transversely through, the air curtain. To supply air to the air knife, it is most typical and economical to use pressure blowers compared to air compressors that require significantly more energy to operate. The most commonly used air knife designs incorporate hollow tubes of various lengths and diameters, with air introduction at either one end, both ends, or the midpoint of the air knife. Air knives channel the blower-driven air through an elongated, single discharge slot opening in a downwardly or outwardly directed curtain of air.
A common problem with air knives is that the volume and velocity of the discharged air can be limited, which in turn can limit the effectiveness of the air curtain, including its effective transverse width, i.e. the width of the curtain in the direction of travel of objects conveyed through the air curtain. Because blower-operated air knives typically direct an elongated, narrow-width air curtain in a straight downward direction, it is typically not possible to effectively apply the air to objects that have irregular surface heights because the air knife is positioned at a fixed distance above the highest surface of the object. To effectively clean, dry or cool at the lowest heights, more air would have to be delivered to reach the lower surfaces which would increase the operating cost of the system.
To overcome the inherent deficiencies of air knives, individual air nozzles are often used to effectively apply the discharge air to surfaces of varying heights. The superior flow characteristics from a properly designed converging nozzle orifice can deliver the blower-driven air to surfaces at a greater distance than the conventional slot type opening typically used in air knife designs. Air nozzles are frequently attached to pipes and manifolds to replace, or augment, conventional air knives. The typical air nozzle manifold system includes externally attached nozzles secured at fixed positions along a pipe or manifold. Although these external fixed nozzle devices provide superior discharge airflow, they do not allow the user to adjust the air output as can be adjusted with an adjustable air knife slot opening. The dimensions of these fixed external nozzle systems can also vary widely to accommodate the various size and shape external nozzle orifices that are attached. These external nozzle manifold systems can be bulky and cumbersome to install, generally requiring additional space, which may not be available, to accommodate the external nozzles when used to replace an air knife with a slot opening.
Another problem that is characteristic to air knives in general is that they produce significant air turbulence as the air exits the elongated slot opening. This turbulence reduces the velocity of the air exiting the elongated slot opening, which also causes the spray pattern to fan out as it exits the air knife's elongated slot opening. The decreased velocity and fan out pattern of the air both adversely affect the performance and effectiveness of the air knife.
Yet another problem that is characteristic to air knives in general is that they employ an elongated slot opening that is fixed with respect to the actual width of the opening through which air is discharged. While the elongated slot opening may be adjustable with respect to its width, there is limited precision with regard to adjusting this dimension. If the gap is opened too wide, large volumes of air must be utilized to maintain the air velocity as the distance from the objects is increased. Alternatively, the system pressure may have to be increased to maintain the velocity of the air at the discharge. Neither is an acceptable alternative because they both require excessive amounts of energy.
Air knives are generally most effective at close proximity to the surface of the objects to be cleaned, dried, or cooled. Because it is not always possible to achieve the ideal air knife positioning relative to the objects, various workarounds have been utilized in an attempt to solve some of the inherent positioning problems when the physical dimensions of the objects to be dried, cleaned or cooled are changed. Unfortunately, these workaround solutions are typically cumbersome, expensive and difficult to implement, and usually result in operational downtime. None of these workaround solutions satisfactorily address the proper and most effective positioning of the air knife relative to the objects to be cleaned, dried, or cooled.
U.S. Pat. No. 6,742,285 to Shepard discloses an air knife that includes an elongated housing having an inlet for receiving air into the housing. The housing includes an elongated gap that extends along the housing that allows air entering the housing through the inlet to exit the housing and form a curtain of air. The elongated housing is made from a piece of sheet metal bent to define a hollow region into which air is forced. The sheet metal defines a gap along a length of the housing from which the air exits. The elongated air knife forms an angle with respect to a direction of travel of objects passing the air knife so that a leading edge of those objects passes progressively different parts of the air knife. Clearly, this type of air knife design does not permit an easy modification to the air discharge portion of the device and would be most suitable when the objects to be cleaned, dried, or cooled are not expected to have changes in their physical dimensions.
U.S. Pat. No. 6,990,751 to Riley et al discloses an air knife or air delivery manifold that uses tangential thrust nozzles to rotate the air knife or delivery manifold to clean or blow off articles of manufacture or other products. The air knife or air manifold is constructed with laterally separated, opposing ends and mounted for rotation about a longitudinal axis. A central inlet opening defines an axis of rotation. The airflow is emitted through a narrow air discharge slot that is rotated over a circular area by jets of air emitted from the thrust nozzles. These air jets rotate the air knife about a longitudinal axis and in a plane parallel to the direction of conveyor advancement. This patent also discloses an alternative system using external nozzles mounted to an air delivery manifold in specific fixed positions to accommodate the rotational features of the device. This type of rotational air knife design would be most suitable when the objects to be cleaned, dried, or cooled have irregular surface features so that air can be applied from different directions. However, it does not permit an easy modification to the air discharge portion of the air knife or provide maximum efficiency of the air knife with respect to optimization of the discharge nozzles. Nor can it easily accommodate increases in the surface height of the objects to be cleaned, dried or cooled without physically raising the device, which would impact the effectiveness of the device on the lowest surfaces unless more air is discharged from the nozzles.
The devices referenced above provide some desirable features and benefits for air knives within the limited scope of their respective designs. However, each has certain obvious drawbacks, as well. Unfortunately, these air knives are typically designed for use in limited applications and are difficult to modify without incurring significant and costly operational downtime.
From the foregoing, it would be desirable to have an apparatus to directionally discharge air that can be easily modified to provide an optimized air stream to accommodate changes in the physical dimensions or irregular surface features of objects that require drying, cooling or cleaning by passing through the air stream. And it would be extremely desirable to have an apparatus that includes uniquely designed discharge air nozzles in a wide range of orifice sizes, shapes, arrays and spacings without requiring any external configuration changes, or complete change out of the apparatus, while at the same time optimizing the efficiency and operating cost of the overall system operation. Furthermore, it would be desirable for the air delivery apparatus to have a low profile, for those applications where the available space between machinery and products is limited.